This invention relates to the field of integrated circuits; and, more particularly, to integrated circuits and the fabrication of integrated circuits including high voltage bipolar transistors.
Bipolar transistors operating at high collector current densities are subject to the Kirk effect, in which the majority carrier charge density associated with the current injected into the collector is equal to the ionized impurity charge density in the collector depletion region, thereby widening the effective base region. Bipolar transistors designed to operate at high voltages (for example, collector-emitter voltages greater than 30 volts) require lower doping densities in the collectors (in the 1014 to 1015 cm−3 range) compared to bipolar transistors designed to operate at less than 10 volts, in order to prevent breakdown of the base-collector junctions. The lower doping densities in the collectors reduce the threshold of collector current density for the onset of the Kirk effect.
High voltage bipolar transistors in integrated circuits (ICs) are typically fabricated with a process sequence designed to fabricate a variety of components, including low voltage bipolar transistors and MOS transistors, at a minimum production cost and complexity. One result of integrating high voltage bipolar transistors into economical IC fabrication process sequences is that the widened base region at high collector current densities due to the Kirk effect extends laterally into a region with a high density of recombination sites, resulting in an increase in base current and an undesirable drop in the gain of the bipolar transistor. Increasing the lateral width of the emitter overlap of the base layer to remove the recombination site region from the collector current path results in an unacceptable increase in base-emitter capacitance.